Pure iron of small grain size



Patented Nov. 12, 1929 UNITED STATES PATENT OFFICE WILHELM OF LUDWIGSHAIEN-ON-TnE-RHINE, WALTER SCH'UBARDT, OF

MANNHEIM, AND OSKAB KRAMER, Q1 OPIEA'U, GERMANY, ASSIGNORS TO I. G, FAR- BENINDUSTRIE AKTIENGESELLSGKAFT, OF FRANKFORT-OLl-THE-MAIN, GERMANY,

A CORPORATION 01! GERMANY PURE IRON OI SMALL GRAIN SIZE.

Ho Drawing. Application filed August 28,1928, Serial No. 802,648, and iu'Germany September 10, 1927.

In the manufacture of iron cores for electromagnetic apparatus, such as transformers, Pupin coils, and the like, use is preferably -made of an iron powder, which has been subjected to heat treatment in a reducing atmosphere and which is comprised of very small and spherical particles. In this heat treatment the small amounts of carbon and oxygen contained in the iron are eliminated, but

sintering occurs at the same time and necessitates regrinding in order to restore to the particles that original small grain size and spherical form which are essential for the said purpose. It has however been found when carrying out this process, in particular on a large scale, that it is not always possible to bring the whole of the material back into its fine condition. Especially when the material has been heated to high temperatures, such as from 500 to 600 centigrade, considerable amounts, as much as 30 per cent and more, of screening residues are left over after grinding, which are not directly applicable for the purpose required; but, on the other hand, the material obtained by the use of high temperatures is just that-which possesses the best electromagnetic properties.

We'have now found that this difiieulty is overcome and the sintering restricted within 30 moderate limits by carrying out the operation in two stages; the powder being first treated with a reducing gas free from carbon such as hydrogen, or ammonia at about 500 centigrade and then, without, if desired, any in-' termedia-te cooling, being heated to temperatures above 500 centigrade but below its melting point with or without the employment of hydrogen or inert gases. The treatment in the first stage decarburizes and deoxidizes the iron, at least partially, whilst in the second stage at higher temperatures the valuableelectromagnetic properties are imparted tothe iron. It has been found that the elimination of carbon and oxygen from the iron proceeds at a sufiiciently rapid rate even at temperatures below 500 centigrade without any substantial sintering taking place.- On subsequent heating to higher temeratures, the iron which has been sufficienty decarburized and deoxidized in this man- Example Iron powder, prepared from iron carbonyl by heat. treatment and containing 1.6 per centof carbon is spread out in a thin layer in a suitable furnaceand is treated with a current of, preferably preheated, hydrogen at temperatures below 500 centigrade until tin-examination of the escaping gases shews that the formation of methane and water, that is the decarburizing or deoxidizing, is substantially complete. The material cooled in an atmosphere of hydrogen still contains about 0.02- to 0.15 per cent of carbon, and can easil be crushed down in a mill so as to leave ehind practically no residue on a sieve with 4900 meshes per square centimetre; This powder, which has a permeability of about 17, is returned to the furnace and is heated at from 500 to 600 centigrade for nitrogen or any other gas which does not produce a chemical change in the iron at high temperatures. After cooling, for example in a current of nitrogen, the mass can be easily ground without leaving any appreciable screen residue, but now has a permeability of between 25 and 32 and, in consequence of its other electromagneticproperties, such as negligible hystereses losses, and the like, is especially adapted for the manufacture of cores for Pupin coils.

It is however-not essential to allow .the

material to cool down, as above mentioned, after decarburizing, and on takin it out of the furnace to return it thereto a ter rinding. On the contrary, as soon as decar buriz ing, by heating in the reducing gas below 500 centigrade has'sufficiently progressed, thetemperature may be raised to from 500 to 600 centi rade and, as already described, maintained -t ereat until the desired electromagnetic properties have been acquired.

The current 'of reducing gas is advanta- 100 geously maintained at a very high velocity, In testimony whereof we have hereunto set since the treatment can thereby be considerour hands.

ably shortened. Also in this case the prod- WILHELM MEISER. I uct is found to be only slightly sintered, after WALTER SCHUBARDT. 5 cooling in a current'of mtrogen, and can be QSKAR KRAMER. 70

easily restored to its original fine globular condition by grinding, as for example in a ballv mill, without leaving any substantial residue 'on screening. n Also it is not essential that the decarburiz- 7 ing process below 500 centigrade shouldbe carried to the extent herein specified, namely as low as to 0.02 to 0.15 percent of carbon, especially when the subsequent rise in temperature above 500 centigrade only proceeds an slowly. The transfer from the first stage to the second can even be effected, while the iron still contains larger amounts of carbon I than those mentioned but not more than 0.5. 4

per cent. 1 a5 The iron powder obtained according to the present invention may valso be usedfor purposes other than the manufacture of Iron cores. For example, it is also excellently adapted for the manufacture of transformer plates, in the radio industry, for coherers, thermal elements, and the like.

What weiclaim is: 1. A process of reducing pure iron of a 30 small grain size w ich comprises treating iron powder with a reducing gas free from [carbon at temperatures between about 300 v C. and about.500 C. until the carbon content is at most 0.5. per cent and then heating in 35 an intert atmosphere to temperatures above 500 C. but below its meltingpoint and grinding the iron.. v

2.-'A process of producing pure iron of small grain size which comprlses tr'eatingiron 40 powder with hydrogen at temperatures be: v tween about 300 C. and about 500 C. until 0 the carbon content is atmost 0.'5 er cent and then heating in'an inert atmosp ere to temperatures above 500 C. but below its melting 45 point and grinding theiron.

3. A process of producing pure'iron of' small grain size which comprises treatin' iron powder with a currentof hydrogen at temperatures between about 300 G. and" a about.500 C.- untilthe carbon content is at most 0.5 per cent, then gradually raising the temperature to between 500 and 600 C. and cooling the iron in a nitrogen atmos phere and grinding it. p I 4. .A process of producing pure iron of smallgrain size. which comprises treating iron powder produced by thermal decomposition ofiron carbonyl with hydrogen at temso peratures between about 300C. andabout 500 C(untiljthe carbon contentis at most [0.5 percent and then heating in an inert a tn iosphere to temperatures above 500 C.

but belowitsmeltin g point and the @65 iron.- 1 

